Set the scene for your character at the beginning of the third season.

When we last saw Brienne she was tied to Jaime Lannister, corralling him to Kingís Landing on the instruction of Catelyn Stark to be exchanged with her two daughters. What we see on that journey is a very difficult relationship evolving - she is constantly being abused by this man who has absolutely no respect for her whatsoever and who continually degrades her. She entirely disrespects him too because in her eyes he is a man without honour.

Why?

Because he is the Kingslayer. Brienne follows a very, very strict moral code, which is the knightís moral code. She has no respect for any knight who breaks it. She believes you have to dedicate your life to that set of moral values. And sheís had to overcome all of the odds to become a knight, so it means even more to her. Itís exclusively a male position - sheís only got there through her dedication, her skill and her power. I think that when someone has that degree of resolution, anyone else that contravenes it she sees as weak and itís a weakness that she has no time for. She is prepared to die for her cause. So thatís our starting point.

How do things evolve as season three progresses?

What Brienne knows as the boundaries of her own reality are really tested - and they are really tested in ways she doesnít expect. As ever in the world of Game of Thrones, you can only ever expect the unexpected and no one is safe. What a character thinks of as their reality changes constantly and obviously the fantasy and fantastical aspects encourage that. Itís a world without limits. Itís a world without parameters and if you think about that in the context of a human psyche, itís explosive. Anything can happen at any time.

How is Brienne tested, as you put it?

All Iíll say is that Brienne has deliberately assumed many male attributes in order to cope in this world. But I donít think as a woman sheís naturally like that. Sheís had to work very hard and behave like a man and only in behaving like a man has she gained any respect from this traditionally very male-dominated world. Itís particularly acute in the environment sheís chosen to put herself inÖ that of being a knight.

She comes from the Island of Tarth. Her father is Lord Selwyn Tarth and thereís an extraordinary back-story in the book about how she became a knight. She never wanted to get married and yet she had to. So she said, ĎIf I manage to beat you in a sword fight I will not marry you.í The man very arrogantly said, ĎWell if I beat you, you will marry me.í We know what happened! Itís actually very similar to the myth of Atlanta in Greek mythology, who had to go through a similar process.

One moment she was serving Renly Baratheon and then the next she was devoted to Cat. How was she able to change her allegiance so suddenly?

You can say that but thatís actually missing out a huge chunk of what happened psychologically. Renly died and the rumours were that Brienne had been accused of Renlyís death. And that isnít what occurred. What occurred was something other-worldly. Now if you take that other-worldly aspect into the world and the decision making process, what is she to do? Remain there and be killed? She says, I should never have fledÖ but she does flee and she flees with a woman who she realises she has an enormous respect for. She sees the way that Catelyn Stark's negotiating the relationship, how smart she is, how strategic she is, but that she has a purity and a sense of whatís right and whatís wrong morally. So Brienne does the next best thing which is to pledge allegiance to someone that has as strict a moral code as she does herself. And also Catelyn Stark's had her daughters taken away from her and no mother deserves that. Brienne believes the best thing she can do is to help return those children.

The dynamic between Brienne and Jaime Lannister is at times hilarious, something like The Odd Couple. Whatís it like working with Nikolaj Coster-Waldau?

The relationship off screen is almost identical! He is mercurial, difficult to deal with, hilarious, brilliantly talented, incredibly kindÖ and heís a great actor. Heís taught me a lot and itís actually been very useful that that sort of dynamic was in place beforehand. Itís enormous fun - a love/hate relationship. Someone that drives you absolutely barking mad! But thatís delicious, too.

You spend all of your screen time wearing armour. Whatís it like?

They make it as real as possible, so itís mostly metal. Itís very heavy, and it is very uncomfortable, but then that is as close as possible, workably, to what she would have worn then. The whole thing about armour is that you canít move with any degree of grace or ease in it. Itís actually very difficult to fight in it. Itís also very difficult to stand up in, and even just walking is a challenge. But thatís what would have been happening to her. If that character was a real person, thatís what she would be having to overcome on a day-to-day basis. It actually says in the books that at night she sleeps in her armour as well because sheís so fearful of being raped. Itís round the clock.